The invention relates to a high critical temperature (HTc) superconductor strand, and to a method of manufacturing such a strand.
The invention relates more particularly to high critical temperature (HTc) superconductor strands manufactured by the xe2x80x9cpowder-in-tubexe2x80x9d (PIT) method.
The PIT technique is known per se and it consists in a first step in densifying HTc superconductor precursors in a metal tube. In a second step, the resulting billet is deformed, e.g. by wire-drawing so as to obtain a xe2x80x9cmonofilamentxe2x80x9d strand. In a third step, the monofilament strand is cut up and packed in a new metal case, thereby forming a multifilament billet. The new, multifilament billet is in turn deformed and shaped so as to obtain a multifilament strand having the desired dimensions and shape. During these manufacturing steps, and in order to transform the precursors into the HTc superconductor phase, it is necessary to perform various transforming heat treatments and various intermediate rolling operations to reactivate the precursors. The material constituting the tube or the case must be sufficiently ductile to be capable of withstanding the various wire-drawing and rolling stages, and it must be of a composition that is inert or at least without consequence for the heat treatment used to transform the HTc superconductor precursors into the superconductor phase. The material must be non-polluting with respect to the precursors and it must be sufficiently permeable to oxygen to pass the oxygen required for proper synthesis of the precursors. It is known that pure silver or a compatible silver alloy mixture (e.g. AgPa) can be used as the material for making the billets.
In the description below, the term xe2x80x9ccompatible silver alloyxe2x80x9d is used to designate a silver alloy that is compatible with the stage of synthesizing the precursors, i.e. an alloy that is non-polluting and that is permeable to oxygen.
Certain applications using HTc superconductor multifilament strands require long continuous lengths (e.g. for superconductor coils). This requires methods of manufacture that enable strands of great length to be obtained in which the electrical and mechanical performance of the strand are maintained along its entire length (and in particular its superconductor phase).
FR 2 752 327 relates to a method of manufacturing an HTc superconductor multifilament strand having a compatible silver alloy matrix, which method is of the powder in tube type and in which:
a square or rectangular monofilament is made having a sheath of compatible silver ally and a core of HTc superconductor precursors;
said monofilament is cut up and the resulting segments are placed in a square or rectangular section compatible silver alloy case, thus making a multifilament billet; and
the heat and mechanical treatments are applied to transform the precursors into the HTc superconductor phase and to obtain the final shape of the multifilament strand.
During manufacture of the multifilament billet, the monofilament segments are arranged layer by layer inside the case, each layer being offset relative to the preceding layer so as to form a configuration of the segment which is staggered in a direction that is perpendicular to the planes of the layers.
The advantages that result from the configuration of FR 2 752 327 are uniform densification of the filaments, a higher deformation ratio of the superconductor phase, and uniform flow of the silver, thereby reducing variations in the section of the strand. As a result, good control is obtained over long strands by reducing the number of defects that can give rise to breakages during manufacture. The compatible silver alloy material used is compatible with the stage of synthesizing the precursors.
Nevertheless, such a strand remains fragile since the compatible silver alloy material used does not have great mechanical strength. As a result the outer sheath of the strand can be torn and that has a significant effect on its properties.
Furthermore, because of the rolling operations, there are portions of the multifilament strand, in particular in the ends of the rectangular section of the strand, in which the density of superconductor filaments is very small. This contributes to poor performance in terms of xe2x80x9cengineeringxe2x80x9d current density Je defined as being the ratio of current flowing in the strand divided by the total cross-section of said strand.
To mitigate the mechanical strength problems of the HTc multifilament strand, it is known to use silver alloys such as AgMg or AgCu.
Nevertheless:
Firstly those materials are polluting for the precursors and therefore can only be used as the outer case of the strand, while its matrix is made of pure silver or of compatible silver alloy (e.g. AgPa).
Secondly, those silver alloys which are required for maintaining the mechanical performance of the strand are not sufficiently permeable to oxygen to guarantee proper synthesis of the superconductor phase of the strand. This gives rise to a strand whose mechanical strength is good but whose electrical characteristics are poor.
In the description below, the term xe2x80x9cnon-compatible silver alloyxe2x80x9d is used to designate a silver alloy that is not compatible with the precursor synthesizing stage, i.e. an alloy that is either polluting or else insufficiently permeable to oxygen.
An object of the present invention is to propose of method of manufacturing an HTc multifilament strand that enables improved mechanical strength, optimum synthesis of the superconductor stage, and increased current density Je all to be obtained simultaneously.
Another object of the present invention is to propose an HTc multifilament strand of great length presenting improved electrical performance and mechanical strength.
To this end, the invention provides a power-in-tube type method of manufacturing HTc superconductor multifilament conductor having a matrix of compatible silver alloy, in which:
a square or rectangular section monofilament is made having a sheath of compatible silver alloy and a core of HTc superconductor precursors;
said monofilament is cut up into segments and a square or rectangular section case of non-compatible silver alloy is filled with the resulting segments, the segments being placed in superposed layers centered in the case of non-compatible silver alloy, each layer being offset relative to the preceding layer so as to form a staggered configuration of the segments;
the gaps left by the offset at the beginnings and ends of the layers are filled with bars of compatible silver alloy, thereby making a multifilament billet that is symmetrical about a superposition midplane; and
mechanical and heat treatments are applied respectively to obtain the final shape of the multifilament strand and to synthesize the precursors into the HTc superconductor phase;
wherein, during preparation of the multifilament billet, the number of segments per layer is organized so that, in cross-section, the segment layers viewed as a whole have the general shape of a converging-diverging nozzle, the throat of the converging-diverging nozzle shape being substantially contained in said superposition midplane; and
wherein, during the mechanical treatment of said multifilament billet preceding the synthesis heat treatment, a rolling force is applied in a direction perpendicular to the superposition midplane, and at least one of the side edges of the rolled billet is cut off perpendicularly to the superposition midplane, thereby revealing the matrix of compatible silver alloy.
Advantageously, to limit the stresses due to rolling, the mechanical treatments include steps of thermally relaxing stresses.
The two edges perpendicular to the mean superposition plane can be cut off.
The invention also provides a multifilament strand having a generally rectangular matrix of compatible silver alloy, comprising a plurality of HTc superconductor filaments each of cross-section that is generally rectangular in shape and arranged in a configuration that is staggered layer on layer, wherein both opposite faces of the rectangular shape are covered in respective layers of non-compatible silver alloy.
A first advantage of the present invention results from the general converging-diverging nozzle shape of the monofilaments in the manufacturing billet. This disposition makes it possible, after the steps of mechanically rolling the strand, to concentrate the monofilaments in the central portion of the strand. Thus, high density and uniformity are obtained in the central portion of the strand, thereby restricting monofilament dispersion to the side edges of the strand.
Another advantage of the present invention results from the combination of using a non-compatible silver alloy material as the outer sheath for the strand and the step of cutting off at least one of the edges of the rolled billet so as to reveal the compatible silver alloy matrix. The non-compatible silver alloy ensures that the strand has good mechanical strength, while uncovering the compatible silver alloy matrix ensures that the precursors are properly synthesized. The result is a strand that has improved mechanical and electrical performance.
An advantage associated with the above advantage is the result of the fact that the excess compatible silver alloy matrix that is poor in monofilaments that results from the rolling and that is located in the side edges of the strand is itself removed during the cutting-off step. This considerably improves current density Je.